1. Field of the Invention
The present invention relates to a steering apparatus for an automotive vehicle capable of effecting normal steerage of the vehicle caused by manual operation of a steering wheel and of effecting automatic steerage of the vehicle for assist of the normal steerage.
2. Description of the Prior Art
Disclosed in Japanese Patent Laid-open Publication No. 6(1994)-206553 is a steering apparatus of this kind which includes an input shaft provided thereon with a steering wheel to be rotated by steering effort applied thereto and to be rotated by a first electric motor with an electromagnetic clutch and a steering shaft connected at one end thereof with the input shaft by means of a planetary gear set and at the other end thereof with a steering gear. The planetary gear set is composed of a sun gear mounted on the input shaft for rotation therewith, a carrier connected to the one end of the steering shafts a ring gear arranged in surrounding relationship with the sun gear to be rotated by a second electric motor and a set of planetary gears supported by the carrier and meshed with the sun gear and ring gear. The electromagnetic clutch of the first electric motor acts to apply a reaction force to the input shaft when disengaged in manual operation of the steering wheel and to retain the input shaft in place when engaged in automatic operation of the steering shaft. The second electric motor acts as a reaction element of the ring gear in manual operation of the steering wheel and acts as a torque input element of the ring gear in automatic operation of the steering shaft.
In the steering apparatus described above, the ring gear of the planetary gear set is rotated by activation of the second electric motor for automatic operation of the steering shaft in a condition where the electromagnetic clutch of the first electric motor is engaged to retain the input shaft in place. If the control system of the second electric motor fails to activate the second electric motor during automatic operation of the steering shaft, there will occur a difference in rotation phase between the steering wheel and the steering shaft. As a result, the steered position of the steering shaft may not be recognized by the driver when the electromagnetic clutch of the first electric motor is disengaged to permit rotation of the input shaft.
Since in the steering apparatus, the electromagnetic clutch of the first electric motor is adapted to apply a reaction force to the input shaft in manual operation of the steering wheel, the control system of the first electric motor becomes complicated in construction to ensure high reliability of the steering apparatus. Since in the steering apparatus, the input shaft is separated from the steering shaft and is operatively connected to the steering shaft by means of the planetary gear set, the steering wheel may not be applied with a reaction force from road surfaces in its manual operation. This deteriorates the steering feel of the driver.